1. Field of the Invention
The present invention relates to a method for frequency matching in an FMCW radar sensor, in which a plurality of frequencies, which are derived on various modulation ramps, and which represent, in each case, an object located by the radar sensor, are shown in a d-v space as geometrical locations which represent possible combinations of distance d and speed v of the respective object, and, in order to identify the objects located on the various modulation ramps, coincidences are searched for between the geometrical locations which belong to frequencies derived on various modulation ramps. The present invention also relates to an FMCW radar sensor in which this method is implemented, and which is able to be used in a driver assistance system for motor vehicles.
2. Description of Related Art
The functional principle of an FMCW radar sensor (frequency modulated continuous wave) is that the frequency of the transmitted radar signal is ramp modulated, and the signal reflected by an object and received again by the sensor is mixed with a part of the signal sent at the receiving time. The mixed product then includes an intermediate frequency component whose frequency corresponds to the difference between the sent and the received signal. This difference is a function, on the one hand, of the object distance, because of the change in the sending frequency that has occurred during the signal propagation time, but on the other hand, it is also a function of the relative speed of the object, because of the Doppler effect.
The intermediate frequency signal is separated by fast Fourier transformation into its frequency spectrum, and each located object is represented in this spectrum by a peak at one frequency, which is a function of the distance and the speed (relative speed) of the object. However, with the aid of this individual frequency, one cannot yet determine unequivocally the actual distance and the actual speed of the object. To do this, it is rather required to locate the same object on at least two modulation ramps of the sent signal, these modulation ramps having to have different slopes.
Each of the two frequencies then represents a multiplicity of possible combinations of distance d and relative speed v of the object. In a d-v space in which the speed of the object is plotted against its distance, the geometrical location of the possible combinations of distance and speed for a given frequency is a straight line whose slope is a function of the slope of the modulation ramp. Thus, for two ramps, one obtains two straight lines having different slopes, and whose intersection, that is, the point, at which the coincidence exists between the geometrical locations associated with the two frequencies, gives the true distance and the true speed of the object.
If more objects are simultaneously located in the locating area of the radar sensor, there is the problem, however, that even in the evaluation of two modulation ramps it cannot be unequivocally determined which peak belongs to which object. For a situation having two objects, one obtains in the d-v space, for example two pairs of parallel straight lines which form four intersections with one another, but only two of these intersections are able to correspond to real objects, while the other intersections represent so-called apparent objects.
In order to obtain unequivocal results, at least a third modulation ramp is required. Real objects may be detected in that coincidence between all three geometrical locations exists in the d-v space, which belong to the frequencies which were obtained on the three different frequency ramps. Graphically this means that all three straight lines, that correspond to the three frequencies, intersect in a point, within the scope of accuracy limits. This coincidence test is designated as frequency matching.
Since, however, the frequencies of the peaks are able to be determined only with limited accuracy, one cannot expect, for a real object, that the three straight lines, that belong to the three modulation ramps, will intersect exactly at one point. One will rather obtain three different intersections, which will lie relatively close to one another, however. Therefore, in order to be able to identify an object at all, a certain tolerance has to be admitted. However, if a large number of objects is located, this tolerance may, in turn, lead to the occurrence of apparent coincidences, so-called false matches, with which no real objects are associated.
In practice, one often works with four different modulation ramps, and the criterion for a real object is that all four straight lines intersect at one point, within the scope of the tolerance limits. Even then, false matches could occur, especially when the number of objects, and thus the density of the peaks in the spectra, is relatively high. The false match rate is proportional to the size of the section of the d-v space, but increases disproportionally with increasing object density and increasing variance of the peak frequencies.
In addition, the computing time required also increases with an increasing number of objects and an increasing number of modulation ramps. Since at least the frequencies obtained on two ramps have to be combined pair-wise with each other, the computing time increases at least quadratically with the number of objects. Such an increase in the computing time is a problem, however, in many applications. This applies, for instance, to the use of an FMCW radar in an adaptive cruise control system for motor vehicles. In that case, the distances and the relative speeds of all preceding vehicles have to be updated in such short time intervals that traffic events may be followed with sufficient accuracy and a distance control is made possible that is appropriate to the situation.
In published German patent application document DE 102 43 811 A1, a method is described for frequency matching in an FMCW radar for motor vehicles, in which the development over time of the distances and relative speeds of the located objects are followed across several measuring cycles of the radar sensor, and false matches in the current measuring cycle are detected with the aid of implausible “jumps” in the dynamic variables of the supposed object. However, this method too requires a great computing effort.